Kovelman R, Russell P
Department of Molecular Biology, Scripps Research Institute, La Jolla, California 92037, USA.
Mol Cell Biol. 1996 Jan;16(1):86-93. doi: 10.1128/MCB.16.1.86.
The DNA replication checkpoint couples the onset of mitosis with the completion of S phase. It is clear that in the fission yeast Schizosaccharomyces pombe, operation of this checkpoint requires maintenance of the inhibitory tyrosyl phosphorylation of Cdc2. Cdc25 phosphatase induces mitosis by dephosphorylating tyrosine 15 of Cdc2. In this report, Cdc25 is shown to accumulate to a very high level in cells arrested in S. This shows that mechanisms which modulate the abundance of Cdc25 are unconnected to the DNA replication checkpoint. Using a Cdc2/cyclin B activation assay, we found that Cdc25 activity increased approximately 10-fold during transit through M phase. Cdc25 was activated by phosphorylations that were dependent on Cdc2 activity in vivo. Cdc25 activation was suppressed in cells arrested in G1 and S. However, Cdc25 was more highly modified and appeared to be somewhat more active in S than in G1. This finding might be connected to the fact that progression from G1 to S increases the likelihood that constitutive Cdc25 overproduction will cause inappropriate mitosis.
DNA复制检查点将有丝分裂的开始与S期的完成联系起来。很明显，在裂殖酵母粟酒裂殖酵母中，该检查点的运作需要维持Cdc2的抑制性酪氨酰磷酸化。Cdc25磷酸酶通过使Cdc2的酪氨酸15去磷酸化来诱导有丝分裂。在本报告中，Cdc25在停滞于S期的细胞中积累到非常高的水平。这表明调节Cdc25丰度的机制与DNA复制检查点无关。使用Cdc2/细胞周期蛋白B激活试验，我们发现Cdc25活性在通过M期的过程中增加了约10倍。Cdc25在体内被依赖于Cdc2活性的磷酸化激活。Cdc25激活在停滞于G1期和S期的细胞中受到抑制。然而，Cdc25在S期比在G1期有更高程度的修饰，并且似乎活性也稍高一些。这一发现可能与从G1期到S期的进展增加了组成型Cdc25过量产生会导致不适当有丝分裂的可能性这一事实有关。
